Diamond and cubic boron nitride (cBN) particles have found widespread use as superabrasives in a variety of abrading and cutting applications. The worldwide consumption of diamond particles currently exceeds 400 metric tons. Common tools which incorporate superabrasive particles include cutting tools, drill bits, circular saws, grinding wheels, lapping belts, polishing pads, and the like. In general, diamond grits can be classified into three distinct size ranges: coarse mesh saw grits (U.S. mesh 18 to 60 or 1 mm to 0.23 mm) for sawing applications, medium sized grinding grits (U.S. mesh 60 to 400, 230 microns to 37 microns) for grinding applications, and fine powder of micron diamond (U.S. mesh <400 mesh) for polishing applications.
Among diamond superabrasives, saw diamond has the largest particle size at about 18 to 60 mesh. High quality saw diamonds are generally euhedral having fully grown crystallographic faces. Further, high quality saw diamond should have very few defects or inclusions. Standard applications for saw diamonds require high quality diamonds. This is at least partially due to the high impact force encountered during cutting, particularly at high speeds. In contrast, smaller diamond particles, i.e. 60 to 400 mesh or 0.25 mm to 37 μm, such as those used in grinding wheels, create scratches in the surface which gradually removes material from a workpiece. In such grinding applications, the impact force is typically much less than for cutting applications. Thus, commercially satisfactory smaller diamonds can be produced with less concern for flaws and impurities than is generally acceptable for larger diamonds such as saw diamonds.
Superabrasives are typically formed under ultrahigh pressure, e.g., about 5.5 GPa and high temperature, e.g., 1300° C. The quality of diamond grits is typically controlled by the growth rate. A slower growth rate can allow for more complete formation of the crystal morphology and a lower amount of interior defects. High quality, well-crystallized diamond grits will exhibit higher impact strength suitable for more aggressive sawing action. The amount of defects (e.g. metal inclusion) will also affect the thermal stability of the diamond grit. A less included diamond can withstand a higher processing temperature (e.g. 1000° C.) typically used for making diamond tools without deterioration. Diamonds having a lower amount of inclusions can also wear slower at the cutting tips where heat is generated.
Diamond grits are typically grown by converting graphite to diamond under catalytic action of a molten metal. The molten metal also serves as a solvent of carbon. Typical catalysts used to synthesis diamond include iron, nickel, cobalt, manganese or their alloys. The growth rate of diamond is controlled by pressure and temperature. Typically, the lower the over-pressure required to make diamond stable and/or the lower the over-temperature needed to melt the catalyst metal, the slower the growth rate. For example, to grow saw grits in a molten alloy of iron and nickel of Invar composition (Fe65-Ni35), the pressure is about 5.2 GPa and temperature is about 1270° C.
Once the growth rate is determined for synthesizing a certain quality of grade of diamond, its size can be determined by the growth time. Because the saw grits are much larger than grinding grits, they require much longer growth time. For example, the growth of 30/40 mesh may require 45 minutes; and 40/50 mesh, 25 minutes. In contrast, the growth of 100/120 mesh may need 2 minutes; and 200/230, 1 minute. Micron diamond is typically produced by pulverizing larger diamond, particularly, larger diamonds with a large amount of defects.
As the time for diamond growth increases, the more difficult it is to control pressure and temperature. However, under ultrahigh pressure conditions during crystal growth, the pressure tends to continually decay due to the volume contraction associated with diamond formation. Further, temperatures within the growth regions can increase due to increases in electrical resistance associated with the diamond formation. Hence, it is very difficult to maintain optimal conditions of pressure and temperature for homogeneous growth of diamond grits. Saw diamond grits are typically grown under ultrahigh pressure over a much longer time, e.g., 40 minutes than that required to grow smaller grinding grits, e.g., about 1 minute. Consequently, saw diamond grits are very difficult to grow, particularly those having high quality. Saw grits with high impact strength are characterized by a euhedral crystal shape and very low inclusions of either metal or graphite. Hence, very tight controls of pressure and temperature are required over extended periods of time to produce high quality diamonds.
These difficulties partially account for the abundance of companies which can grow saw grits, while very few companies are capable of growing high grade saw grits having larger sizes. As a result, very few companies can master the technology of growing coarse saw grits, in particular, those with high quality, high impact strength, and high thermal stability.
Typical methods for synthesizing larger high quality diamonds involve ensuring uniformity of raw materials such as graphite and metal catalyst and carefully controlling process temperature and pressures. High pressure high temperature (HPHT) processes used in diamond growth can employ reaction volumes of over 200 cm3. Most often, the graphite to diamond conversion in the reaction volume can be up to about 30%. Unfortunately, typical processes also result in the crystals having external flaws, e.g., rough surfaces, and undesirable inclusions, e.g., metal and carbon inclusions. Therefore, increased costs are incurred in segregating acceptable high strength diamonds from weaker, poor quality diamonds.
One major factor to consider in diamond synthesis of high grade saw diamonds is providing conditions such that nucleation of diamond occurs uniformly and nearly simultaneously. Random nucleation methods typically allow some regions of raw materials to be wasted while other regions are densely packed with diamond crystals having a high percentage of defects. Some diamond synthesis methods have improved nucleation uniformity somewhat; however, during diamond growth local changes in pressure can occur. If heating is accomplished by passing electrical current directly through the reaction cell, then diamond growth can also interfere with the electrical current used to control heating. The results of such interference are non-uniformities and fluctuations in the temperature and pressure gradients across the reaction cell and thus a wide distribution of crystal sizes, crystal shapes, and inclusion levels. Despite these difficulties, by providing highly homogeneous starting materials and carefully controlling process conditions, the volume efficiency of the reaction cell is still typically less than 2 to 3 carats per cubic centimeter. This marginal yield still wastes large amounts of raw materials, reduces production efficiencies, and leaves considerable room for improvement.
Other methods for synthesizing large industrial diamond particles include forming layers of solid disks of graphite and/or catalyst. Diamond nucleation then occurs at the interface between graphite and catalyst layers. However, such materials are intrinsically heterogeneous. For example, the firing temperature for graphite rods that are cut into disks can vary from region to region, thus affecting the microstructure and composition of the disk. Further, during mechanical formation of graphite into a rod, the graphite microstructure can change, e.g., the outer regions exhibit a skin effect during extrusion. As a result, graphite disks tend to have regions which vary in porosity, degree of graphitization, ash content, and the like. Similarly, catalyst disks have varying alloy composition as metal atoms and crystal structure tend to segregate during cooling. Additionally, during extrusion and mechanical forming processes the alloy composition in various regions changes even further. As a result, local concentrations and properties of graphite and catalyst metals can vary by several percent across solid disks. Diamonds grown under such conditions tend to nucleate at different times and experience varying growth rates, thus producing diamonds having a wide size distribution and increasing the number of flawed diamonds due to intergrowth, overgrowth, i.e. fast growth rates, and uneven growth, i.e. asymmetric growth, as shown in FIGS. 19A, 19B, and 22.
Recently, efforts have been made in using powdered materials to further increase yields of industrial diamond particles. These methods attempt to uniformly mix graphite and catalyst powders to achieve improved diamond nucleation. However, diamond nucleation still occurs randomly, i.e. broad size distribution, but somewhat uniformly throughout the powder under HPHT conditions, as shown in FIG. 23. Such methods have met with some success and have resulted in improved yields of up to 3 carat/cm3. Further, yields of high quality diamond of specific sizes have also improved up to five times over those achievable using conventional layered disk methods. However, powdered mixture methods can be difficult to control. For example, the density of graphite and metal catalyst materials differ significantly, making uniform mixing very difficult. In addition, powdered mixture methods generally require even more strict control of process conditions than in layered methods.
Therefore, methods which further increase the quality and yields of large diamond particles suitable for commercial use continue to be sought through research and development.